Mouse peritoneal macrophages from inflammatory foci show a catalogue of characteristics which distinguish them from unstimulated cells, including (a) elevated pinocytic rate, (b) increased rate of synthesis and secretion of neutral proteases (plasminogen activator, elastase, collagenase), (c) ingestion of IgM-C3-coated particles, (d) low or absent levels of 5' nucleotidase activity. These characteristics will be used as criteria for judging the activation of macrophages in vivo and in vitro by lymphocytes responding to antigen, allogeneic cells, or tumor cell surfaces, in order to establish the mechanism of macrophage activation by cell-mediated immune reactions.